There is known a developer container that stores toner and has a toner filling port formed therein. The toner filling port is generally closed by a toner cap so as to prevent the toner in the developer container from leaking therefrom through the toner filling port. The toner cap has an insertion portion to be inserted into the toner filling port and a flange portion protruding radially outward from one end of the insertion portion. The flange portion of the toner cap is welded onto an outer surface of the developer container to close the toner filling port while preventing the toner cap from coming off from the toner filling port.
Downsizing of the developer container having the above configuration may pose a problem that a sufficient space for welding the flange portion of the toner cap onto the outer surface of the developer container cannot be ensured. In such a case, the toner cap may come off to cause the toner to leak.